1. Field of the Invention
The present invention relates to a DC-to-DC converter, and particularly to control of an output voltage from a DC-to-DC converter.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing a conventional DC-to-DC converter. Descriptions will be provided for an operation of the DC-to-DC converter shown in FIG. 1. To begin with, once a voltage is applied from a DC power supply Vin, an activation circuit (not illustrated) causes a control circuit 10 to start its operation. The control circuit 10 includes an oscillating circuit 11, a D-type flip-flop circuit 13, dead time generator circuits 14, 15, a level shifter circuit 16, and buffer circuits 17, 18. The control circuit 10 alternately turns on and off switching elements Q1, Q2 with a dead time.
Once the switching element Q2 is turned on, an electric current flows from and to the DC power supply Vin through the switching element Q2, a reactor Lr, a primary winding P and a current resonance capacitor Cri. This current is a combined current of: an excitation current flowing through an excitation inductance Lp on the primary side of a transformer T; and a load current supplied to a load from output terminals +Vo, −Vo via a primary winding P, a secondary winding S2, a diode D2, and a capacitor Co. The former current is a resonance current shaped like a sine wave, and produced by (the reactor Lr+the excitation inductance Lp) and the current resonance capacitor Cri. The former current has a low resonance frequency in comparison with an on-time of the switching element Q2, and thereby is observed as if a part of the sine wave were a triangular wave. The latter current is a resonance current shaped like a sine wave in which an element of resonance between the reactor Lr and the current resonance capacitor Cri appears.
Once the switching element Q2 is turned off, energy of the excitation current stored in the transformer T causes a voltage pseudo resonance among (the reactor Lr+the excitation inductance Lp), the current resonance capacitor Cri, and a voltage resonance capacitor Crv. At this time, a resonance frequency produced by the voltage resonance capacitor Cry with a small capacitance is observed as a voltage across each of the switching elements Q1, Q2. To put it specifically, once the switching element Q2 is turned off, the current of the switching element Q2 moves to the voltage resonance capacitor Cry. Once the voltage resonance capacitor Cry is discharged to zero volts, the current moves to a diode D8. The energy of the excitation current stored in the transformer T charges the current resonance capacitor Cri through the diode D8. If the switching element Q1 is turned on during this time period, the switching element Q1 is capable of operating as a zero-volt switch.
Once the switching element Q1 is turned on, a current flows from and to the current resonance capacitor Cri through the primary winding P, the reactor Lr and the switching element Q1 with the current resonance capacitor Cri being used as its power supply. This current is a combined current of: an excitation current flowing through the excitation inductance Lp of the transformer T; and a load current supplied to the load from the output terminals +Vo, −Vo via the primary winding P, the secondary winding S1, a diode D1, and the smoothing capacitor Co. The former current is a resonance current shaped like a sine wave, which is produced by (the reactor Lr+the excitation inductance Lp) and the current resonance capacitor Cri. The former current has a low resonance frequency in comparison with an on-time of the switching element Q1, and thereby is observed as if a part of the sine wave were a triangular wave. The latter current is a resonance current shaped like a sine wave in which an element of resonance between the reactor Lr and the current resonance capacitor Cri appears.
Once the switching element Q1 is turned off, energy of the excitation current stored in the transformer T causes a voltage pseudo resonance among (the reactor Lr+the excitation inductance Lp), the current resonance capacitor Cri, and the voltage resonance capacitor Crv. At this time, the resonance frequency produced by the voltage resonance capacitor Cry whose capacitance is small is observed as a voltage across each of the switching elements Q1, Q2. To put it specifically, once the switching element Q1 is turned off, the current of the switching element Q1 moves to the voltage resonance capacitor Cry. Once the voltage resonance capacitor Cry is charged up to the voltage of the DC power supply Vin, the current moves to a diode D9. The energy of the excitation current stored in the transformer T is regenerated in the DC power supply Vin through the diode D9. By being turned on during this time period, the switching element Q2 is capable of operating as a zero-volt switch.
FIG. 2A is a diagram showing waveforms observed when the voltage of the DC power supply Vin is 300V and the DC-to-DC converter is at 100% load (heavy load). FIG. 2B is a diagram showing waveforms observed when the voltage of the DC power supply Vin is 400V and the DC-to-DC converter is at 100% load. Comparison between FIG. 2A and FIG. 2B shows that the resonance current Icri flowing through the current resonance capacitor Cri is almost constant irrespective of whether the input voltage is large or small. However, in the waveforms under the lower input voltage, the frequency is lower, and the excitation current is larger. As step-up energy, part of this excitation current is stored in the reactor Lr, and largely contributes to the raising of the output voltage. The output voltage can be controlled by controlling the switching frequency. The output voltage is detected by an output voltage detector circuit 20, and the detected output voltage is transferred to a feedback terminal (FB terminal) of the control circuit 10 on the primary side of the DC-to-DC converter through a photocoupler PC. Thus, an oscillation frequency of the oscillating circuit 11 is regulated according to the output voltage. Note that: FIG. 3A is a diagram showing waveforms observed when the voltage of the DC power supply Vin is 300V and the DC-to-DC converter is at 0% load (no load); and FIG. 3B is a diagram showing waveforms observed when the voltage of the DC power supply Vin is 400V and the DC-to-DC converter is at 0% load.
FIG. 4 is a diagram showing a relationship between an output power ratio and the switching frequency. As learned from FIG. 4, when the load is changed from heavy load to no load, the switching frequency needs to be increased almost four times from approximately 40 kHz to approximately 160 kHz. In recent years, DC-to-DC converters have been designed to operate at higher frequencies for downsizing. Particularly, such resonant DC-to-DC converters are circuits suitable to operate at higher frequencies because of their low switching loss.
Note that, for instance, a current-resonance converter described in Japanese Patent Application Publication No. 2005-39975 has been known as related prior art.
However, the frequency at no load needs to be approximately four times higher than the frequency at maximum power. For this reason, even the resonant DC-to-DC converter is incapable of coping with the rise in the frequency. For instance, if a circuit for a DC-to-DC converter is designed to operate at a frequency of 2 MHz at maximum load, the maximum frequency would go up as high as 8 MHz. For this reason, such a DC-to-DC converter cannot be put to practical use in consideration of factors such as electric power consumed to drive the switching elements and power loss caused by the control circuits. In the above case, the resonant DC-to-DC converter has to be used with the highest and lowest frequencies of approximately 2 MHz and 500 kHz, respectively.
Furthermore, if the highest and lowest frequencies are set high as described above, the range between the highest and lowest frequencies extend beyond a frequency range of AM radio broadcast. This makes it difficult to take EMC (electromagnetic compatibility) measures.